Tracking Disease Through Mosquito Slobber

This is nothing to spit at: Scientists say they may be able to track deadly mosquito-borne diseases by studying the saliva the insects leave behind when they feed on sugary bait.

Mosquito-borne diseases are a major health hazard worldwide. Some, like malaria, chronically afflict certain regions. But others, such as dengue fever, West Nile virus, and chikungunya, can rapidly emerge in new locations or reappear in areas where they've gone dormant. That means public health officials must keep a constant eye on the diseases' movement.

The usual methods for detecting mosquito-borne viruses all have a weakness: Relying on clinical diagnoses means a disease has already arrived in the population; keeping ''sentinel'' animals is costly, and the animals themselves provide a food source for mosquitoes; and capturing thousands of mosquitoes and analyzing their RNA is expensive and labor-intensive.

Now, Andrew van den Hurk, a virologist with Queensland Health Forensic and Scientific Services in Coopers Plains, Australia, and colleagues have found a way to monitor mosquito-borne diseases that may be simpler than current methods and suitable for use over large geographic areas. For their new study, the researchers took advantage of the fact that mosquitoes are sloppy eaters: When they feed on a sugar source, the insects leave behind a slobbery mess. And van den Hurk and colleagues have found that they can detect viruses in that residue of mosquito spit, as they report online today in the Proceedings of the National Academy of Sciences.

To prove it, the scientists created box traps that lure mosquitoes with carbon dioxide gas—mosquitoes are attracted to the stuff because it indicates the presence of a breathing animal and therefore a meal—and then suck them inside with a fan. Once inside the trap, the mosquitoes feed on filter paper soaked with honey—dyed blue so that the color rubs off on the mosquitoes that take the bait. The researchers set out traps in Bunbury in Western Australia and near Cairns, in Northeastern Australia—two historical hot beds of the mosquito-borne Ross River and Barmah Forest viruses. Over 11 weeks, scientists returned to the traps weekly to collect the filter paper and trapped mosquitoes and send them to the lab for analysis.

Whenever the filter paper returned positive results for viral RNA, the labs also found the viruses present in the mosquitoes they'd captured, meaning the filter paper accurately reflected the presence of virus-carrying mosquitoes. The scientists suggest the technique might be able to be modified to detect other diseases like malaria and bluetongue virus.

Honey is antibacterial, so it's an excellent medium for protecting viral RNA from bacteria until researchers return to collect it. The traps can be left out for more than a week, allowing them to set out traps over a relatively large geographical range and check the traps intermittently, the researchers say. Another benefit is speed. Labs can analyze the filter paper using an RNA-identifying technique known as reverse transcription polymerase chain reaction, which reveals whether the diseases are present almost instantaneously. By contrast, sorting, preserving, and transporting trapped mosquitoes for RNA analysis is a much lengthier and more laborious process. The current method also requires keeping the mosquito samples cold—something that's not always feasible in hot, tropical environments.

Jonathan Day, an entomologist at the University of Florida's medical entomology lab in Vero Beach, says that, compared with analyzing trapped mosquitoes or finding infection in sentinel animals, the new technique could essentially cut in half the time it takes to detect and respond to an outbreak in a new area,. But he says it remains to be seen whether the technique will be cost-efficient enough to warrant researchers switching to it. ''It certainly is clever,'' Day says, ''but cost is the critical factor.'' Day also points out that the technique lets you know only whether the disease is present and cannot tell researchers how widespread the infection is among an area's mosquito population.

Rory McAbee, a biologist with the Fresno Mosquito and Vector Control District in California, agrees that the method could be a timesaver for researchers, but cautions that the traps will only work for mosquitoes that are attracted to carbon dioxide. ''Not all species are going to go into the trap,'' she says. ''This would have to be evaluated for each species and virus.''